1. Field of the Invention
The present invention relates to a wet friction material and a process for producing the same. More particularly, the present invention relates to a friction material for clutch for use in automatic transmission, particularly a wet friction material suitable for use in an oily liquid, and a process for producing the same.
2. Description of the Related Art
As clutches for use in transmission for vehicle such as automobile and train there are widely used friction clutches. The friction material to be incorporated in this kind of friction clutches is normally obtained by a process which comprises subjecting a fiber base material such as natural pulp fiber, organic synthetic fiber and inorganic fiber, a filler such as diatomaceous earth and cashew resin and a friction adjusting agent to wet paper making process to form a paper, impregnating the paper with a thermosetting resin such as phenol resin, and then compression-molding the paper under heating.
These friction materials can be divided into two types, i.e., dry friction material which is used in dry form and wet friction material which is used in an oily liquid. Friction materials suited for respective purposes are developed.
As the thermosetting resin to be incorporated in these friction materials, there are used various thermosetting resins. A phenol resin is widely used from the standpoint of heat resistance, abrasion resistance, handleability, cost, etc. As such phenol resins, there are used phenol resins which has been modified with various resins to improve various properties. There are many kinds of such modified phenol resins. As one of these modified phenol resins, there is proposed a phenol resin which is obtained by bonding a straight-chain organopolysiloxane to a novolak phenol resin to exhibit improvements in elasticity, tension transmission characteristics and tensile strength (JP-A-55-92738 (The term "JP-A" as used herein means an "Unexamined Japanese Patent Publication (Kokai)")).
However, such a phenol resin modified with a straight-chain organopolysiloxane is disadvantageous in that it exhibits a lowered softening point or a reduced stability of organopolysiloxane. In order to obtain a binder that provides better heat resistance and abrasion resistance, a proposal was made to use as a binder a compound mainly composed of a phenol resin, as one of the foregoing modified phenol resins, modified with an organopolysiloxane containing R.sup.1 SiO.sub.1.5 unit (in which R.sup.1 represents the same or different C.sub.1-8 substituted or unsubstituted monovalent hydrocarbon group) and/or SiO.sub.2 unit to produce a friction material (JP-A-61-192711).
Further, it is proposed in a clutch facing comprising as constituents a fibrous material, an abrasion-resistant powder, a metal wire or metal powder and a binder that as such a binder there be used a silicone resin in an amount of from 0.1 to 10% by weight based on the weight of the clutch facing in combination with a conventional binder such as phenol-formaldehyde resin, melamine-formaldehyde resin, melamine-phenol-formaldehyde resin, SBR, NBR and natural rubber (JP-A-60-28484).
In accordance with the arrangement thus proposed, a clutch facing which can hardly absorb moisture can be obtained. However, this proposal says that the silicone resin to be used for this purpose is not specifically limited so far as it can exert an effect of preventing moisture absorption. In some detail, only dimethylsiloxane silicone resins are exemplified in the above cited patent application. There is no detailed consideration of the technical content.
Then, it has been proposed a wet friction material comprising a thermosetting synthetic resin made of a silicone resin represented by the following average composition formula, the amount of trifunctional unit monomers (RSiO.sub.3/2 in which R represents a substituted or unsubstituted monovalent hydrocarbon group) in monomers constituting the silicone resin being from 40 to 100 mol-% (JP-A-7-197016): EQU (R.sup.1).sub.a (OR.sup.2).sub.b SiO.sub.(4-a-b)/2
wherein R.sup.1 represents a substituted or unsubstituted monovalent hydrocarbon group; R.sup.2 represents a hydrogen atom or C.sub.1-10 organic group; and the suffixes a and b represent a positive number satisfying the relationships 0.2.ltoreq.a.ltoreq.1.8 and 0&lt;b.ltoreq.1.9, as a wet friction material which can provide a wet friction material having an excellent heat endurance and a high torque transmission capacity to meet the demand for the control over slip in friction clutch or the reduction of friction in ATF (automatic transmission fluid) in order to reduce fuel cost and improve feeling during transmission in the modern automobile automatic transmission from the standpoint of energy problem or environmental problem.
The wet friction material thus developed exhibits an excellent heat resistance, a high durability and a high torque transmission capacity as expected. However, if the foregoing wet friction material containing a silicone resin is produced by the conventional production process which comprises subjecting a fiber based material and a filler in admixture to wet paper making process to form a paper, impregnating the paper with a thermosetting resin, and then compression-molding the paper under heating, the resulting flexibility becomes too great due to the characteristics of the foregoing silicone resin. As a result, the wet friction material thus produced occasionally leaves something to be desired in shearing strength, peeling life, etc.
Even if the foregoing production process is conducted by mixing a phenol resin as a thermosetting resin with a silicone resin in an attempt to attain both the desired strength and flexibility for wet friction material, a phenol resin and a silicone resin can hardly be mixed with each other as generally known.
A production process has then been proposed which comprises mixing a powdered phenol resin with a powdered silicone resin, a filler and a fiber base material to obtain a paper, and then applying heat to the paper to obtain a wet friction material (JP-A-7-173301). This wet friction material is obtained by mixing the foregoing powdered resins. By applying heat to the paper thus obtained, these resins harden. This production process involves no step of impregnating the paper with these resins, making it possible to avoid complicatedness in mixing uniformly two liquid resins.
However, a wet friction material having an excellent shearing strength and heat resistance can be obtained by the conventional production process which comprises subjecting a fiber based material and a filler in admixture to wet paper making process to form a paper, impregnating the paper with a thermosetting resin, and then compression-molding the paper under heating rather than the foregoing proposed production process.